Engine components that are exposed to the hot combustion gas flow of modern combustion turbines are required to operate at ever-increasing temperatures as engine efficiency requirements continue to advance. Ceramics typically have higher heat tolerance and lower thermal conductivities than metals. For this reason, ceramics have been used both as structural materials in place of metallic materials and as coatings for both metal and ceramic structures. Ceramic matrix composite (CMC) wall structures with ceramic insulation outer coatings, such as described in commonly assigned U.S. Pat. No. 6,197,424, have been developed to provide components with the high temperature stability of ceramics without the brittleness of monolithic ceramics.
Further as to the relatively lower thermal conductivity of CMCs, it is known to use radiation cooling, such as described in commonly assigned U.S. Pat. No. 6,767,659, and/or convective cooling or impingement cooling on back surfaces of component walls. However, backside cooling efficiency is reduced by the low thermal conductivity of ceramic material and by the fact that the wall thickness of a CMC structure, to achieve a desired strength, may be thicker than an equivalent metal structure. U.S. Pat. No. 5,687,572 teaches a backside impingement-cooled cylindrical ceramic liner of a combustor attached by pins to an outer metal shell. This reference cites thicknesses expected to withstand particular loads, discusses that thinner liners have lower thermal stresses, and refers to an analysis of buckling. It does not deviate from a uniform cylindrical configuration of the ceramic liner.
More generally, the issues related to strength properties per unit weight or thickness and to the cooling of structures made with CMCs are of particular concern for gas turbine engine components that are exposed to or are near the hot combustion gas path. As one approach to address these issues, a CMC lamellate wall structure with a high temperature ceramic insulation coating, commonly referred to as friable grade insulation (FGI), is disclosed in commonly assigned U.S. Pat. No. 6,197,424. Current materials of this type provide strength and temperature stability to temperatures approaching 1700° C. Also, the commonly assigned U.S. Pat. No. 6,709,230 describes cooling channels in a ceramic core of a gas turbine vane behind an outer CMC airfoil shell, and commonly assigned U.S. Pat. No. 6,746,755 uses ceramic matrix composite cooling tubes between CMC face sheets to form a CMC wall structure with internal cooling channels.
Notwithstanding these advances, further improvements in the design of hybrid CMC/ceramic insulating layer apparatuses are desired to support further applications of such structures in gas turbine engines, particularly in those engines in which an increase in the firing temperatures is expected and/or greater loads are imposed on the transition.